Several methods are known for the preparation of 2-oxocarboxylic acid esters. Among such methods are (1) oxidation of an acetylene compound with osmium oxide (See, Journal of Organic Chemistry. 43, 21 (1978); 4245); (2) reaction of an oxalic acid diester with an alkyl aluminum (See, German Patent Publication No. 2151867); (3) oxidative dehydrogenation of a 2-hydroxycarboxylic acid ester in gas phase (See, Japanese Patent Laid-Open Publication No. Hei 5-255190 and Japanese Patent Laid-Open Publication No. Hei 8-34762); (4) oxidation of a 2-hydroxycarboxylic acid ester with chlorine under light irradiation (See, Japanese Patent Laid-Open Publication No. Hei 11-228502); (5) oxidation of a 2-hydroxycarboxylic acid ester with a hypochlorite in a hydrophobic solvent in the presence of a hindered secondary amine under acidic condition of pH 6 or lower (See, Japanese Patent Laid-Open Publication No. Hei 11-315052); (6) oxidation of a 2-hydroxycarboxylic acid ester with a hypochlorous acid in the presence of a catalytic amount of a nitroxyl radical in a solvent substantially consisting of a halogenated hydrocarbon (See, Japanese Patent Laid-Open Publication No. Hei 3-58956); and (7) oxidation of a 2-hydroxycarboxylic acid ester with sodium hypochlorite in the presence of a nitroxyl radical and sodium bromide (See, Tetrahedron Letters. 33, 35 (1992); 5033).
These approaches, however, all have their respective disadvantages: the process (1) involves the use of osmium oxide which is harmful to human being; the process (2) involves the use of an alkyl aluminum, a material difficult to handle on an industrial scale; the process (3), which is a gas phase reaction, and the process (4), which involves light irradiation, each requires special facilities; in the process (5), the hindered secondary amine must be activated prior to use by treatment with a peracid, which is difficult to handle; the process (6) involves the use of a halogenated hydrocarbon solvent, which is harmful to environment; and by the process (7), the desired 2-oxocarboxylic acid ester can be produced only at low yield. Thus, none of these approaches is an industrially advantageous process for the preparation of 2-oxocarboxylic acid esters.
Accordingly, it is an objective of the present invention to provide an industrially advantageous process of 2-oxocarboxylic acid esters that allows efficient, simple preparation of the compounds.